This invention relates to an internal combustion engine of the stratified combustion type provided with a pre-combustion chamber which ensures effective combustion of a lean fuel-air mixture.
It is known that the most effective method of reducing the amounts of carbon monoxide (CO), unburnt hydrocarbons (HC) and nitrogen oxides (NOx) contained in exhaust gases discharged from internal combustion engines is to burn a fuel-air mixture which is as lean as possible. However, it is necessary to overcome various difficulties as pointed out below in order that a very lean mixture having a large air-fuel ratio of, for example, 20 can be stably burnt within the cylinder. One of the difficulties encountered in the combustion of a lean mixture having such a large air-fuel ratio in a known gasoline engine is that misfire tends to occur due to the fact that such a lean mixture has a very low ignitability. This ignitability is further lowered especially when residual gases produced as a result of combustion of such lean mixture remain in the vicinity of the discharge electrode of the ignition plug. Combustion of such lean mixture encounters such difficulty that the efficiency of the engine is lowered and the rate of fuel consumption is increased due to the fact that the rate of combustion of a very lean mixture is quite low.
Internal combustion engines of the stratified combustion type are known which overcome such difficulties and are capable of ensuring sustained combustion of a lean mixture without giving rise to undesirable misfire. As is already well known in the art, these stratified combustion engines are designed on the basis of a unique combustion theory. According to this theory, a rich mixture is formed in a limited zone in the vicinity of the ignition plug to be initially ignited by the spark of the ignition plug so as to provide an igniting source or touch, and this igniting source or torch is utilized to cause combustion of a lean mixture surrounding the rich fuel-air mixture. A stratified combustion engine, which is provided with a pre-combustion chamber in addition to a main combustion chamber, is known as one type of such stratified combustion engines. The manner of such combustion employed in this type of stratified combustion engine is such that a rich mixture is charged in the pre-combustion chamber to be initially ignited by the spark of the ignition plug and then this combustion flame is forced through a communication passage into the main combustion chamber with an increase in the pressure due to the combustion of the fuel-air mixture in the precombustion chamber so as to utilize this combustion flame as an igniting source or torch for causing combustion of a lean fuel-air mixture in the main combustion chamber. The communication passage extending between the precombustion chamber and the main combustion chamber in the engine of this type has a relatively small sectional area. Therefore, the burning flame is jetted from the precombustion chamber into the main combustion chamber at a considerably high speed while spreading over a wide range so that the lean mixture in the main combustion chamber can be immediately ignited by the burning flame. Further, a very strong turbulent flow is produced in the main combustion chamber by the combustion flame flowing into the main combustion chamber at high speed, and the rate of combustion is increased by the turbulence thus produced. (This effect is commonly called the "torch" effect.)
The stratified combustion engine of the type above described is relatively old and is well known in the art. Such an engine has at each cylinder a main combustion chamber, a precombustion chamber which communicates through a communication passage with the main combustion chamber, a main suction passage for supplying a lean mixture into the main combustion chamber, a rich mixture supply conduit disposed in the main suction passage, and a main inlet valve common to both the main suction passage and the rich mixture supply conduit. The engine design has as its objective the aim of reducing fuel comsumption, but it has not been put into production up to the present time because of its more complex structure as compared to that of a conventional engine and because of its disadvantages which include:
1. The rich mixture in the precombustion chamber is diluted with the lean mixture forced into the precombustion cyamber upon the compression stroke of the engine, thereby tending to misfire;
2. As charging of the rich mixture into the precombustion chamber is effected through the main combustion chamber, the rich mixture is interfered with by the lean mixture which results in poor charging efficiency for the precombustion chamber;
3. Scavenging of the burnt gas in the precombustion chamber cannot be fully effected during the suction stroke of the engine because of interference of flow of the rich mixture with the burnt gas and the lean mixture.
4. As the scavenging of burnt gas in the precombustion chamber and charging of the rich mixture into the precombustion chamber is effected through only one communication passage, interference of the burnt gas with the rich mixture occurs. This also leads to poor charging efficiency for the precombustion chamber and to misfire.
5. When the spark electrode is disposed in the vicinity of a communication passage which provides communication between the precombustion chamber and the main combustion chamber, the mixture existing in the vicinity of the spark electrode tends to be diluted with the lean mixture forced into the precombustion chamber, thus resulting in misfire. On the other hand, if the spark electrode is disposed at the innermost position in the precombustion chamber, the mixture existing remote from the spark electrode is discharged in the unignited state from the precombustion chamber by the expanding pressure of the ienited mixture existing in the vicinity of the spark electrode. This makes it impossible to obtain a sustained torch.
To eliminate these disadvantages, a modified stratified combustion engine has been developed recently and put into mass production. In such modified engine, the precombustion chamber is directly connected to a carburetor and is provided with a sub-inlet valve for the suction of the rich mixture. The main combustion chamber, on the other hand, is connected only to the main suction passage for the lean mixture supply. A main inlet valve is disposed in the main suction passge, and the rich mixture supply conduit is eliminated.
In this modified stratified combustion engine, the sub-inlet valve is opened at the end of the compression stroke of the engine (that is, immediately before ignition); therefore, it is possible to prevent dilution of the rich mixture with the lean misture. In addition, scavenging of burnt gas in the precombustion chamber can be fully effected because of a lack of interference of rich mixture charging flow with lean mixture in the main combustion chamber during the suction stroke of the engine.
Such modified stratified combustion engine is substantially effective for achieving stable combustion of an extremely lean mixture, but, on the other hand, it has the disadvantage that a complex valve mechanism and a cylinder head having a complex structure are required.
In addition, the modified stratified combustion engine includes the disadvantage (5) set forth above.
A further modification of the engine of the modified type has also been proposed, in which the sub-inlet valve provided in the precombustion chamber is substituted for a fuel injection valve. This further modified engine, however, also has the disadvantage that the structure of the fuel supplying system is complex and the manufacturing cost is also considerably increased due to the fact that a special controller is required for accurately controlling the quantity of fuel to be injected and fuel injection timing.
There is, therefore, a considerable demand for a further improved stratified combustion engine of simple construction.